A known image forming apparatus, such as a color laser printer, may include a belt unit having a belt for conveying paper or intermediate transfer. The belt unit may include, for example, a belt driving roller rotatably driven and a belt driven roller rotated in a driven manner, both of which being supported by a synthetic resin belt frame. The belt unit belt is extended between the rollers. The belt driven roller has belt guides at both ends, and protrusions (guide ribs) provided at side ends of an inner surface of the belt, the protrusions engaging the belt guides to position the belt in a width direction. The belt frame supports a plurality of transfer rollers for transfer of a developer image to recording medium, and the transfer rollers are placed to face, via the belt, a plurality of photosensitive drums supported by a body frame in an apparatus body.
Such a belt unit is, for example, urged in an axial direction of the belt driving roller (the width direction of the belt) by driving a gear (helical gear) connected to the belt driving roller when mounted in the apparatus body, and an end of a shaft of the belt driving roller is abutted against the body frame in the apparatus body to position the entire belt unit in the width direction of the belt.
In such an image forming apparatus, relative positional accuracy of each photosensitive drum and each transfer roller and paper needs to be ensured for obtaining stable image quality without color shift. Thus, the belt for conveying paper requires positional accuracy relative to each photosensitive drum. However, in the above configuration, the belt is positioned in the width direction relative to the body frame via many components (such as the belt driven roller, the belt frame, and the belt driving roller), and positional accuracy in the width direction is hard to ensure.
Thus, there is a need in the art for an image forming apparatus that allows positional accuracy of a belt to be ensured and allows stable image quality to be obtained.